The invention offers an improvement to the method for making heat-dissipating elements for microelectronic devices disclosed in U.S. Pat. No. 5,413,751 Polese et al., which patent is incorporated herein and made part of the present disclosure by this reference.
The aforesaid patent taught a novel and non-obvious process illustrated in FIG. 1 in the manufacture of heat-dissipating components by powder metallurgy. The process began with preclustering 10 the metal particles of metal having different densities and melting points to yield a free-flowing composite powder that allowed direct pressing 11 and sintering 12 of a powder compact 13 into the near-shape of the desired component 14. The invention avoided the time-consuming and expensive processes of cutting and machining the components from a composite block made by the prior art copper-infiltration process as disclosed in U.S. Pat. No. 4,680,618 Kurada et al. However, components made according to the aforesaid method have been found to exhibit surface accumulation of copper. It appears that during the sintering process, copper bleeds out of the outer layer of the compact due to an excessive capillarity phenomenon and concentrate on the periphery of the component. The excess copper tends to cause plating blisters during the brazing process, severely affect the coefficient of thermal expansion at the surface of the heat-dissipating component.
In spite of the grinding and lapping, remaining porous surface areas from which the copper had bled away would occasionally cause pitting during the plating process.
It was found necessary to remove the excess copper by abrasion and even grind away the subcutaneous layer from which the bleeding copper had been drawn. Thus, the heat-sink element could only be pressed and sintered to a near-net shape, preferably slightly oversize. Their top and bottom surfaces must then be ground 15, their sides lapped 16 down to net shape and holes and cavities 19 machined 17 before plating 18.
The instant invention results from a search for a more efficient process that avoids the surface bleeding of the copper.